Numerous applications currently exist for reinforced plastic resins, such as automobiles, off-the road vehicles, and boats, airplanes, and other vehicles. The resins, such as polyester, epoxides, and phenolics, form the matrix which is strengthened by reinforcing means, such as glass or carbon fibers. Mixtures of the resin and reinforcing means are often referred to as "bulk molding composition" or "sheet molding composition" which are formed into the desired part by methods such as compression molding techniques.
One of the desired characteristics of the sheet molding compositions as compared to the ceramic and metal materials having comparable physical characteristics is that the sheet molding compositions are lower in density. This is a substantial advantage in the efficiency of a vehicle made using this material. However, it remains an advantage to produce even lower density SMC. On the other hand, in many applications it is desirable that the SMC has a high gloss surface in order to reduce wind resistance, and to provide an enhanced appearance.
The present method utilizes hollow filler to reduce the density of the composition in which it is incorporated. At the same time, the method results in a high gloss finish through the application of a film to a heated surface. As an aspect of the invention, the heating is applied primarily to a surface portion of the substrate, i.e., the substrate is not heated to a high temperature throughout the thickness. Consequently, the substrate is less likely to release gas. Since gas which is trapped during molding tends to be trapped below a substrate surface, the surface layer, generally has less trapped gas since the gas can more easily escape. In contrast to the present invention, the prior art relates to a high temperature preheating treatment of a sample for a period of time sufficient to allow the substrate to degas throughout. This process is longer and therefore more expensive. More particularly, the film is conductive and the method includes an electrostatic application of a second coating onto the first film. Alternatively, other thermal means could be used to heat a surface such as directed hot air. For large parts, heating by means of infrared heating may be preferable.
U.S. Pat. No. 5,021,297 relates to a process of coating a preformed plastic substrate susceptible to gassing upon heating in which the substrate is preheated at a temperature and for a time sufficient to degas the substrate, which temperature is sufficient to cure a subsequently applied powder coating; and then immediately applying a powder coating to the preheated substrate at a temperature sufficient to cure the powder coating; and then heating the coated substrate so as to cure the powder coated composition.